In a matrix screen color CRT, a large number of phosphor dots are formed on the inner surface of the CRT's glass display screen using photolithography. Trios of the phosphor dots, generally surrounded by a black matrix of non-luminescent material for improved contrast, provide the primary colors of red, green and blue when a raster-scanned electron beam is incident thereon. To ensure that the intended electron beam is incident upon only designated phosphor dots, an apertured color selection electrode, or shadow mask, is positioned between the electron beam source and the display screen, with the electron beams passing through the large number of apertures in the shadow mask before being incident upon the display screen's phosphor dots. Precise registration between shadow mask apertures and display phosphor dots is critical to ensure a high degree of brightness uniformity and color purity of a video image presented on the display screen.
In the photolithography process, a light source typically in the form of a high pressure, water cooled mercury lamp in combination with an optical aperture directs light through the shadow mask apertures onto the display screen's inner coated surface. In forming the phosphor dots on the display screen, it is important that the light directed through the shadow mask apertures closely simulate electron beam trajectories to ensure proper registration between each shadow mask aperture and its corresponding phosphor dots. It is also important for shadow mask aperture-phosphor dot registration that the light source appear as a point light source. The finite dimensions of the light source and the inability to closely approximate electron beam trajectories by the path of the light directed onto the display screen have given rise to residual misregistration errors between the shadow mask apertures and phosphor dots. In high resolution computer display CRTs, this residual misregistration is even greater than that in a conventional color television receiver CRT giving rise to even greater reductions in the levels of color purity and white uniformity on the computer display.
Referring to FIG. 1a, there is shown a simplified side elevation view of a typical prior art light source apparatus 10 used in the formation of phosphor dots on the inner surface of a display screen 34 of a color CRT. The light source apparatus 10 includes a housing 12 having an aperture through which light is directed from a light source 16. The light emanating from light source 16 is shown in simplified form as a pair of linear rays 24a and 24b directed through respective apertures in a shadow mask 36 positioned adjacent to the inner surface of the CRT display screen 34. Light rays 24a, 24b pass through the aforementioned aperture in an upper portion of housing 12, where the CRT display screen 34 is disposed over the aperture. Positioned within housing 12 is a lamp house 14 in which is disposed a light source 16 and a slot or gap disposed above the light source. This latter structure is shown in greater detail in the perspective view of FIG. 1b which shows the linear, elongated, cylindrical shaped light source 16 disposed adjacent to first and second covers or shielding 18a and 18b. The first and second covers 18a, 18b form a generally semi-circular optical slot or aperture 20 therebetween for directing the light through a flat cover glass 22. Cover glass 22 is generally cylindrical in shape and is attached to lamp house 14 by means of an O-ring seal 28 in combination with a generally circular positioning bracket 26. After exiting lamp house 14 and transitting the flat cover glass 22, the light passes first through a trimmer 30 and thence through a correction lens 32 before passing through the apertures within shadow mask 36 and then onto the phosphor-bearing material on the inner surface of the display screen 34. As shown in the figures, the x-axis is horizontal and generally defined as coincident with the longitudinal axis of the cylindrical light source 16, the y-axis is horizontal and extends perpendicular to the longitudinal axis of the light source, and the z-axis passes through the light source and is oriented perpendicular to the plane defined by the x- and y-axes.
Various approaches have been adopted in attempting to improve shadow mask aperture-phosphor dot registration. One such prior approach is disclosed in Japanese Patent Document 2-260351 which employs an optical aperture having a general V-shaped cross section. The width of the aperture gradually narrows from its center to its sides to allegedly provide better grading of phosphor dot size. This optical aperture thus is intended to correct for phosphor dot size errors rather than for residual misregistration. Another approach is disclosed in U.S. Pat. No. 5,270,753 which employs an optical aperture generally parabolic in shape and constructed of a series of segments having dissimilar slopes to provide a slot which is not bilaterally symmetrical. This later approach, while somewhat correcting for residual misregistration exhibits high registration sensitivity, thus requiring precise optical alignment and mechanical assembly of the light source and associated components. These characteristics limit the commercial attractiveness of this approach.
The present invention addresses the aforementioned limitations of the prior art by providing a light source including a uniquely configured optical aperture which corrects over the entire area of the CRT's display screen for residual misregistratation errors arising from the effects of a self-convergent magnetic deflection yoke on the trajectories of the electron beams. The inventive light source also substantially reduces registration sensitivity of the light beam directed onto the display screen and relaxes tolerances in the mechanical assembly and alignment of the light source.